1. Technical Field
The present invention relates to a shock sensor with bistable mechanism and to a method of shock detection.
2. Description of the Related Art
On many occasions, it is useful to know whether a given device or object, such as, for example but not only, a portable electronic device, has undergone shocks of an intensity such as to cause potential damage. This type of information may prove useful for various reasons and may make it possible, among other things, to facilitate diagnostic procedures, offer a guide to the choice of the most appropriate maintenance intervention and verify that the use of the device has been in compliance with the specifications. Likewise, it is possible to check, for example for insurance purposes, whether an object entrusted to a delivery carrier has suffered damage during transport.
Generally, for such purposes as these it is not necessary to measure precisely all the stresses to which the device has been subjected, whereas it is desired to keep track of the occurrence of a potentially harmful impact. More precisely, the important information regards whether at least one impact of intensity higher than a given threshold has occurred or not.
Several kinds of sensors are known, in particular of a microelectromechanical type, which may be used in monitoring the impact undergone in the course of the service life of a device.
Active sensors, based for example on MEMS (microelectromechanical systems) accelerometers, are extremely accurate and reliable and thus enable storage of large amounts of precise data. However, for many applications such a high level of accuracy is not needed and it is sufficient to store a single event. The cost of active sensors is consequently not justified. Moreover, the power consumption of active sensors is considerable and may significantly affect the autonomy of the devices in which the sensors themselves are incorporated.
Passive shock sensors, on the other hand, are inexpensive and the power consumption is negligible or even zero, but in general the control of the threshold is somewhat approximate. For example, the document EP 1 394 555 and the equivalent document U.S. Pat. No. 6,858,810 describe a semiconductor shock sensor, which comprises a supporting body and a movable mass, connected to the supporting body by suspension springs. The supporting body and the movable mass are moreover connected by sample elements, defined by elastic conductive elements (in particular, made of doped polycrystalline silicon) having weakening regions. When the sensor undergoes an acceleration, the mass translates according to the degrees of freedom allowed by the system of suspension springs and subjects the sample elements to stress. If the acceleration exceeds a threshold along at least one axis, the weakening regions of the sample elements break. The state of the sample elements (intact or interrupted) may be easily determined by applying a voltage at the ends of the sample elements themselves: if the sample elements are intact, the voltage causes a current to flow, whereas the current is zero if the sample elements are interrupted. The shock sensor is relatively simple and inexpensive to produce, can be easily integrated in a semiconductor chip and then incorporated in any electronic device and requires an electric power supply exclusively for reading. However, the threshold for ultimate tensile strength of the sample elements can be determined only with a degree of approximation that is frequently unsatisfactory, both because the dynamics of the breaking processes are largely unforeseeable and the process spread cannot be eliminated. The accuracy of this type of shock sensors can hence prove insufficient.
Other types of shock sensors, described in US 2011/0100124, are based upon a chamber containing a conductive liquid and having a wall configured to break following an impact of intensity higher than a given threshold. In this case, the liquid comes out into a second chamber and closes the contact between two electrical terminals. The same document describes also shock sensors that comprise a magnetic element held suspended by a conductive spring between metal walls. Following upon an impact of sufficient intensity, the magnetic element comes into contact with one of the two walls and also in this case brings about closing of an electrical circuit that comprises the magnetic element, the spring and the walls.
In either case, the structures are rather complicated and it is desired to use materials that are far from suited to being integrated in processes for manufacturing semiconductor microelectromechanical devices. The shock sensors described in US 2011/0100124 envisage in fact structures encapsulated in chambers, which are problematical if not impossible to produce with the techniques of manufacture of integrated circuits. In particular, it is problematical to provide a conductive liquid in a closed chamber contained inside another chamber. Moreover, also the accuracy of the shock sensors remains in any case unsatisfactory. In one case, in fact, the force threshold is determined by breaking of a diaphragm and in the other case, the force threshold is determined by the balance between magnetic forces and elastic return forces and hence depends upon the geometry, the characteristics of the spring and the process spread. All the types of shock sensors are in any case far from suited to being incorporated in electronic devices, especially portable ones.